A dictionary mapping integer file descriptors to instances of FileDescriptor
which have been registered with the reactor. All
FileDescriptors which are currently receiving read or write
readiness notifications will be present as values in this dictionary.

A dictionary mapping integer file descriptors to arbitrary values (this is
essentially a set). Keys in this dictionary will be registered with
_kq for read readiness notifications which will be dispatched
to the corresponding FileDescriptor
instances in _selectables.

A dictionary mapping integer file descriptors to arbitrary values (this is
essentially a set). Keys in this dictionary will be registered with
_kq for write readiness notifications which will be dispatched
to the corresponding FileDescriptor
instances in _selectables.

A flag which indicates whether any signal handlers will be installed during
startup. This includes handlers for SIGCHLD to monitor child processes,
and SIGINT, SIGTERM, and SIGBREAK to stop the reactor. (type: bool)

A flag which is true between the time reactor.stop is called
and the time the shutdown system event is fired. This is used to determine
whether that event should be fired after each iteration through the
mainloop. This should be replaced with an explicit state machine. (type: bool)

A flag which is true from the time reactor.run is called until
the time reactor.run returns. This is used to prevent calls
to reactor.run on a running reactor. This should be replaced
with an explicit state machine. (type: bool)

A dictionary mapping integer file descriptors to instances of FileDescriptor
which have been registered with the reactor. All
FileDescriptors which are currently receiving read or write
readiness notifications will be present as values in this dictionary.

_reads =

A dictionary mapping integer file descriptors to arbitrary values (this is
essentially a set). Keys in this dictionary will be registered with
_kq for read readiness notifications which will be dispatched
to the corresponding FileDescriptor
instances in _selectables.

_writes =

A dictionary mapping integer file descriptors to arbitrary values (this is
essentially a set). Keys in this dictionary will be registered with
_kq for write readiness notifications which will be dispatched
to the corresponding FileDescriptor
instances in _selectables.